Method for producing a homogeneous biomass fuel for gasification applications

ABSTRACT

A method for producing a gasifier feedstock with uniform gasification characteristics from a number of biomass sources which each exhibit differences in their drying, pyrolysis, devolatization or gasification characteristics. The method allows biomass from a wide range and sources and with a wide range of gasification characteristics to be utilized within a gasifier without modification of the gasifier unit. The method produces a fuel pellet with controllable size, density and moisture content. The method smoothes any changes in calorific value which may exist between different feedstocks. It has been found that the gasification characteristics of a pellet formed in this way is strongly dependant upon the shape, moisture content and density of the pellet formed and less sensitive to the origin of the biomass feedstock.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of co-pending U.S. Provisional Patent Application No. 60/785,933, filed Mar. 24, 2006, entitled METHOD TO PRODUCE A HOMOGENEOUS BIOMASS FUEL FOR GASIFICATION APPLICATIONS, and commonly assigned to the assignee of the present application, the disclosure of which is incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to the production of a homogenous gasifier fuel feedstock with uniform gasification characteristics from a number of biomass sources which each source exhibits differences in their drying, pyrolysis, devolatization or gasification characteristics.

BACKGROUND OF THE INVENTION

Gasification processes convert carbon-containing solids or liquids into combustible gases that ideally contain all the energy originally present in the feed. In reality this is not easily achieved, although with good thermal management it is possible to operate with energy efficiencies in excess of 90%. Extensive research has been conducted into the area of biomass gasification. The technique yields a combustible gas from a biomass solid. Gaseous fuels have many advantages over solid fuels as they can be utilized with controllable heat realization rates and devices can be started up or shut down very rapidly. Biomass is a very broad definition and includes all solids derived from plant matter as well as organic municipal waste. Suitable biomasses include, but are not limited to, sawdust, wood, straw, alfalfa seed straw, barley straw, bean straw, corn cobs, corn stalks, cotton gin trash, rice hulls, paper, municipal solid waste, barks and animal wastes and combinations of the foregoing. It is interesting that almost all biomass has the same approximate ratio of carbon to hydrogen to oxygen, which is summarized as CH_(1.4)O_(0.6). The stoichiometric gasification equation is shown below: CH_(1.4)O_(0.6)+0.2O₂→CO+0.7H₂  (1)

An energy balance across Equation 1 reveals that the products contain more energy than the reactants, hence some of the biomass is burnt to offset this imbalance. Although the chemical composition of all biomass is similar their gasification behaviors are not. Biomasses from different sources can have vastly different drying and devolatization characteristics leading to vastly different charcoal-like structures. These different gasification behaviors, coupled with different physical structures, have caused difficulties for gasifier designers in that design modifications are required for almost every type of biomass utilized. This can be illustrated by considering the classic Imbert design. The Imbert design is a vertical gasifier consisting of a straight biomass holding section followed by a tapered section which terminates at the throat. Nozzles are situated just above or at the tapered section to allow for the introduction of oxidants. The angle and final throat diameter dictates the final throughput capability of the unit. Due to vastly different biomass behaviors the design allows that the tapered throat be interchangeable with throats of different tapers and final diameters. When the unit is being commissioned various throats are tested until favorable gasifier performance is attained.

The use of biomass as a feedstock for gasification systems is becoming increasingly economically as well as environmentally attractive. The syngas produced by gasification should ideally be used locally, utilizing the fuel to produce power, using the fuel to offset natural gas in heating applications or to convert the syngas into a liquid fuel are all viable options. The conversion of the syngas to a liquid fuel can be readily accomplished by the catalytic reduction of carbon monoxide by hydrogen to produce methanol, ethanol or synthetic middle distillates. In this case the fuel can be readily transported to be the market place.

A typical biomass has an energy density of approximately 18 kJ/g on a dry basis. On a wet basis this value can be substantially less and can even be less than zero, indicating that the fuel is not capable of burning in a sustainable manner while liberating energy. On a dry basis biomass has a calorific value about half that of coal. The low energy density, its low packing density and difficulty in handling make the economics of transporting biomass large distances unfeasible. Thus, the utilization of biomass for small to medium scale distributed energy producing processes has some synergy. The biomass for such a process would be sourced locally and probably within a twenty mile or so radius. Power may be generated and used to reverse feed already saturated power delivery lines. In such a system local communities would utilize locally grown biomass and potentially make use of some volume of waste currently being land filled to generate their own power or convert the material into fuels. In effect, a community could become power and fuel self-sufficient while producing essentially no or nominal greenhouse gas emissions.

The technique of gasifier fuel densification also promotes the transportation characteristics and handling of biomass. During densification the inter- and intra-particle void spaces are vastly reduced in volume. The resulting solid is much smaller and therefore requires less volume for transportation. The technique allows for particles of approximately the same characteristic dimension to be created.

SUMMARY OF THE INVENTION

The invention comprises, in one embodiment thereof, a method for producing a gasifier fuel with uniform gasification characteristics from a number of biomass sources which each exhibit differences in their drying, pyrolysis, devolatization and char oxidation characteristics. The combination of these differences leads to large variations in their gasification behavior. Moreover, the invention relates to a method of producing a homogeneous fuel by grinding and mixing each biomass constituent, drying and then forming a solid pellet through the application of pressure and/or a binding agent.

The preparation method allows biomass from a wide range and sources and with a wide range of gasification characteristics to be utilized within a gasifier without modification of the gasifier unit, while maintaining the gasifier throughput rate and thermal profile. The method produces a fuel pellet with controllable size, density and moisture content. The method smoothes any changes in calorific value which may exist between different feedstocks. It has been found that the gasification characteristics of a pellet formed in this way is strongly dependant upon the shape, moisture content and density of the pellet formed and less sensitive to the origin of the biomass feedstock. The method allows for large variations in the ratios of biomass components. The method also allows fuels which may otherwise be problematic, for example due to ash content, to be blended with biomasses which do not exhibit the same issue to produce a fuel which can then be readily used.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Biomass comes in many forms, including woody biomass such as forest products and the wastes derived from wood including saw dust and paper and cardboard products. Non-woody forms of biomass include animal wastes, decomposable municipal waste, food production waste and energy crops. In both cases the biomass is an organic matter of recent origin. The physical form and typical associated moisture contents of the biomasses described above are vastly different, however, on dry ash free basis the chemical composition and calorific value of the fuels above are similar. Traditionally, using a wide range of range of fuels in gasification device has proved difficult, even though chemically speaking the biomasses are similar; this is attributed to differences in the gasification behavior of different biomass materials. Thermogravimetric studies have found rates of biomass gasification vary over an order of magnitude depending upon the source and type of biomass. One possible explanation is that associated minerals in the biomass may act to catalytically aid or retard the thermal depolymerization which occurs during devolatization and pyrolysis.

In gasification system the ability to operate with a number of different fuels offers tremendous advantages such as the ability to purchase cheap and readily available feedstocks. This ability minimizes the biomass stock piles as seasonally available fuels can. This ability is particularly attractive for biomass fuelled systems where the economics do not allow for the biomass to be transported long distances.

In the present invention a biomass preparation method is described which allows a wide range of fuels to be combined to produce a homogeneous fuels. The method results in a compacted solid which is designed to impose a slight heat and mass transfer limitation, such that the pellet reacts at the rate of heat and mass transfer and not at the rate dictated by the biomass composition. Thus, even as the ratios of the constituent components are changed the gasification properties remain similar.

The biomass may be processed using a number of commercially available pieces of equipment. In some cases it may be preferable to initially air dry the biomass by leaving the biomass exposed to air and sun. This method can dramatically reduce the inherent moisture contents of a number of biomasses. Alternatively, the coarse biomass can be crudely dried by force venting feed silos, possibly with the application of heat. Once the feed is dried to the desired level the biomasses are pulverized using a shredding, hammering chipping or other device or technique known to those skilled in the art. It is possible to combine the different types of biomass at any point through out the process, although extremely satisfactory results may be achieved through the blending after the pulverizing stage. The biomass is reduced in size such that the characteristic particle length is substantially smaller than that of the final fuel pellet. After size reduction the biomass may be further dried. Rotary driers may be utilized with satisfactory results. Densification can be performed a number of ways with extrusion pelletizers or briquetting machines being cost effect techniques. The final fuel pellet may be transferred to a storage silo prior to gasification.

The economics of the process may be improved through the use of waste heat produced during the gasification process or by the utilization of the gas produced from the gasification process. An example would be to utilize waste heat to dry and/or preheat the biomass to the predetermined levels.

Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. It should further be noted that any patents, applications and publications referred to herein are incorporated by reference in their entirety. 

1) A method for producing a homogeneous gasifier fuel from a plurality of different biomasses, wherein each biomass exhibits differing drying, pyrolysis, devolatization or gasification characteristics, comprising: a) drying each type of biomass to a moisture content in the range of from about 0% to about 50%; b) reducing the characteristic length of the biomass particles to the range of from about 0.01 inches to about 0.1 inches; c) combining and mixing the different types of biomass together; d) densification of the biomass to produce a solid with a characteristic length in the range of from about 1 cm to about 5 cm; e) densifying the biomass such that the particles contains a voidage fraction less than about 25%; and, f) feeding the pellet into a suitable gasification designed for the size, shape, void fraction and moisture content of a fuel described in a) through e) above. 2) The method of claim 1, wherein a binding agent is added prior to densification to aid in particle coagulation. 3) The method of claim 1, wherein a cubic structure is formed with a characteristic length in the range of about 0.5 cm to about 8 cm. 4) The method of claim 1, wherein a cubic structure is formed with a characteristic length in the range of about 1 cm to about 3 cm. 5) The method of claim 1, wherein a cylinder-like structure is formed having a characteristic diameter of from about 0.1 cm to about 3 cm and having a characteristic length of from about 0.5 cm to about 3 cm. 6) The method of claim 1, wherein a cylinder-like structure is formed with a characteristic diameter of from about 0.2 cm to about 1 cm and having characteristic length of from about 0.6 cm to about 1.2 cm. 7) The method of claim 1, wherein the biomass is steam treated to improve the compaction qualities of the cellulostic matter. 